A generally used type of lithographic printing plate precursor (by which we mean a coated printing plate prior to exposure and development) has a radiation sensitive coating applied to an aluminum substrate. A positive working precursor has a radiation sensitive coating, which after imagewise exposure to radiation of a suitable wavelength becomes more soluble in the exposed areas than in the non-exposed areas, in a developer. Only the remaining, image, area of the coating is ink-receptive.
The differentiation between image and non-image areas is made in the exposure process where a film is applied to the printing plate precursor with a vacuum to ensure good contact. The printing plate precursor is then exposed to a radiation source; conventionally this has been a UV radiation source. In the case where a positive printing plate precursor is used, the area of the film that corresponds to the image in the printing plate precursor is opaque so that no light will strike the printing plate precursor, whereas the area on the film that corresponds to the non-image area is clear and permits the transmission of light to the coating which becomes more soluble and is removed on development.
In the manufacture of electronic parts such as printed circuits, after exposure to radiation and development, the resist pattern is used as a mask for forming the patterns onto the underlying electronic elements--for example by etching an underlying copper foil. Due to the high resolution demands and the requirements of high resistance to etching techniques, positive-working systems are widely used. In particular, in the main there have been used alkali developable positive working resists mainly composed of alkali-soluble novolac resins.
The types of electronic parts whose manufacture may use a resist include printed wiring boards (PWBs), thick- and thin-film circuits, comprising passive elements such as resistors, capacitors and inductors; multichip devices (MDCs); and integrated circuits (ICs). These are all classified as printed circuits.
Imagable compositions may also be applied to plastics films in order to form masks. The required pattern is formed on the mask, which is then used as a screen in a later processing step, in forming a pattern on, for example, a printing plate or electronic part precursor.
Common to virtually all commercial applications of positive working systems employing UV radiation over several decades have been compositions comprising alkali soluble phenolic resins and naphthoquinone diazide (NQD) derivatives. The NQD derivatives have been simple NQD compounds used in admixture with resins, or NQD resin esters in which the photoactive NQD moiety has been chemically attached to the resin itself, for example by esterification of the resin with an NQD sulfonyl chloride.
U.S. Pat. No. 3,802,885 describes a UV sensitive positive working printing plate containing a naphthoquinone-(1,2)-diazide-(2)-5-sulphonic acid derivative, the printing life of which is said to be improved by the inclusion of a polymeric carboxylic acid. Polymeric carboxylic acids listed are cellulose acetate hydrogen phthalate, collophony-containing resin, carboxyl group containing styrene-maleic acid copolymer, oil-free alkyd resin, fatty acid-free phthalate resin and poly(vinyl hydrogen phthalate). Example 1 of U.S. Pat. No. 3,802,885 describes a number of compositions each containing a polymeric carboxylic acid, a novolac resin and 2,3,4-trihydroxy benzophenone tris-[naphthoquinone-(1,2)-diazide-(2)-5-sulphonate]. Each such composition was tested as a printing plate coating and found to have an estimated life ("run length") of more than 200,000 copies. A comparison composition without a polymeric carboxylic acid failed after 20 revolutions due to poor adhesion of the image to the plate surface.
The naphthoquinone compounds of U.S. Pat. No. 3,802,885 are known as 215-NQD compounds; the moiety .dbd.O is at the 1-position, the moiety .dbd.N.sub.2 is at the 2-position and the moiety --SO.sub.2 --X is at the 5-position (thus, on the adjacent fused ring of the naphthyl group).
Digital and laser imaging technology is now making new demands on coatings. We have devised new positive working heat sensitive systems, to meet the new demands. In one important development described in WO 99/01796 we determined that heat could image coatings containing diazide moieties, without causing lysis of the diazide moieties. Heat can be delivered to the coatings described in WO 99/01796 by conduction, using a heated body such as a stylus, or by charged particle radiation, or, preferably, by means of infra-red radiation, the coatings then containing suitable infra-red absorbers.
It would be desirable to increase the operating speed of imagable articles, but this generally results in articles with lower chemical resistance--by which we mean resistance to organic chemicals, notably those typically used in printing processes and in PCB manufacture. Equally, it would be desirable to improve chemical resistance, but this generally results in a reduction in operating speed. In practice a compromise must be reached, and often the compromise is disappointing.
By "operating speed" in this specification we mean the criterion which is alternatively known in the art as "sensitivity"; the question of how much energy is needed to effect imaging, given also the developer conditions and other parameters selected. Thus, when we mention "operating speed" in this specification we are considering this in the context of the entire process of exposure and development. We are not referring only to the matter of how the areas of the composition which are exposed react to that exposure.
It is an object of embodiments of this invention to provide articles with imagable coatings which have improved chemical resistance yet still with good operating speed; or to provide articles with imagable coatings with improved operating speed, yet still with good chemical resistance; or to provide articles with improved operating speed and improved chemical resistance.